4 Claims. (Cl. 23-143) This invention relates to the production of alumina from clays. More particularly the invention is concerned with the production from an iron-containing clay of a `high grade, substantially silicaand iron-free alumina suitable as feed in the electrolytic aluminum reduction process.

YBauxite, the principal ore of alumina (aluminum` oxide-A1203), has heretofore been, and is presently, the main source of alumina as feed in the electrolytic production of aluminum. Clay has long been recognized as another possible source of high grade alumina, and various proposals have heretofore been advanced for producing such alumina from clays. In clay, alumina is combined with silica (SiO2), generally in the form of a hydrous silicate of alumina (eg. Al2O32SiO2-2H2`O). Clays further contain varying amounts of other constituents, `frequently referred to as impurities, such 'as excess silica, iron compounds, e.g. ferrie oxide (uFeZOg), etc. The present invention contemplates an improved method of producing a high grade alumina product subtantially Afree of silica and iron, from clays containing varying amounts of silica and iron.

The method of the invention involves two stages of treatment in the first of which silica is removed from the cl-ay and a crude alumina containing ferrie oxide is produced, and in the second of which the fer-ric oxide is eliminated and a high `grade alumina product is produced. Basically, the first stage treatment involves calcining the clay at an elevated temperature to render the alumina and iron soluble in hydrochloric acid (HC1). The calcined clay is leached with an aqueous solution of hydrochloric acid, preferably of a concentration of about 20% HC1,

to solubilize the aluminum and `iron compounds as chlorides in an aqueous solution leaving practically all of the silica in the insoluble leach residue. Following filtration, or other suitable solids-liquid separatory step, the solution of aluminum and iron chlorides is concentrated by evaporation to produce a crystal slurry or magma of aluminum and iron chlorides This crystal slurry, after suitable conditioning if necessary or desired, is calcined at an elevated temperature in the course of which the `chlorides are decomposed into oxides with the evolution of hydrogen chloride (hydrochloric acid gas) which is recovered for reuse in the treat- 'ment of further calcined clay. The calci-ne is a substantially silica-free crude alumina product containing viron oxide usually analyzing about 85% (plus or minus 5 A1203 and (plus or minus 5%) Fe203, depending on the quality of the clay feed, and constitu-tes the feed of the second stage treatment.

u The purpose of the hydrochloric acid extraction, or rst stage, treatment is to produce a crude :alumina product, free of silica, for a succeeding purification, or second stage, treatment. Metallurgically the 4alumina and iron oxide in the calcined clay are soluble in hydrochloric acid, while silica is insoluble. The chloride-sof ice aluminum Iand iron (after separation from the silica residue and suitable dehydration) are amenable tofdecornposition by thermal treatment to yield hydrogen `chloride gas and a crude alumina product substantially free of silica but contaminated with iron oxide in proportion to its occurrence in the clay. The purpose of the purilcation, or second stage, treatment is to remove the contaminating iron oxide from the crude alumina. Because the crude alumina is substantially free of silica, it is admirably amenable `to either of two` puriiication treatments for eliminating the iron oxide, which treatments are hereinafter referred to as the caustic .soda digestion treatment and the `soda-sinter treatment. While a minor amount of silica can be tolerated in the caustic soda digestion treatment, its 'presence is undesirable since itleads to loss of alumina. In the soda-.sinter treatment, the crude alumina should be substantially free of silica because silica would be solubilized and might thus contaminate tthe desired l'high grade alumina product. The choice of purification treatment depends to some .extent upon the iron content of the clay, the caustic soda digestion treatment being generally more advantageous where the rion content of the clay is relatively'low., `say not QX- ceeding about 6% `(calculated `as Fe203), while the sodasint'er treatment is generally more advantageous where 'the iron content of the clay is relatively high.

In each purication treatment the primary objective is to solubilize the alumina content of the crude product `as an aqueous solution of sodium aluminate (NaAlOg) without substantially altering .the Water-insolubleproperty of the iron oxide in the crude product. The insoluble iron oxide residue is separated 'from the sodium aluminate solution. The solution is appropriately treated .to precipitate aluminum hydroxide (AMOI-U3), and the `precipitate` is c-alcned `to produce substantially silicaand Viron-free alumina suitable as feed to the pot line of an electrictrolytic aluminum reduction plant.

Whenthe purication treatment involves caustic soda digestion, the thermal treatment of the slurry of aluminum and iron chlorides is carried Aout ot a temperature within the rangeof 1000 and 1300 F., `and while the resulting crude alumina consists ,principally of the oxides lof aluminum and iron (eg. in the relative proportions of 3- 6 to l), it may, and usually does, contain a minor amount of undecomposed chloride, probably as ferrie chloride. VThe crude alumina is digested Ain ,an aqueous caustic s oda (NaOH) liquor at elevated temperature ,and pressure, in the vcourse of which sodium -aluminate is formedin solution while the iron is retained in the insoluble nesidue mainly as ferrie oxide. The sodium aluminate solution 'is separated from the insoluble residue, the ysolutionws appropriately 'treated to precipitate aluminum hydroxide, and the precipitate is calcined to produce substantially silicaand iron-freelalurnina.

`When the purification .treatment involves soda-sintering, the thermal treatment of 'the slurry of aluminum and iron chlorides is carried out at a `temperature `Within .the range of 1500 and 1900* F., Aand the resulfmrg crude alumina is for all practical )purposes free of chlorine. .ln

this purification treatment, the crude .alumina siutered in 1tliepresenceof sodium carbonate or soda .ash `(N azCQs) lat a temperature of .at 'least 1500u F. (,:and Ypreferably 1.700-1900" R), in 'the course of which lthe oxide is converted into water-sluble sodium faluminate while the "iron is retained :in the resulting caleine as 'insoluble ferrie oxide. For convenience this .operation :is herein designated` the soda-sinter step. VThe s'inter :is leached to dissolve the sodium aluminate and theresulting pregnant solution is separated from the insoluble leach residue consisting mainlyof ferrie oxide. "The solution of sodium'alurninateispreferablytreated with car'bpn dioxide gas Vin the 'caurse oflwhieh 'aluminum hydroxide is precipitated and sodium ycarbonate is regenerated for reuse in the soda-sinter step.l The precipitated aluminum hydroxide, after washing if necessary or desired, is dehydrated by calcination at and elevated temperature to produce'a high grade, substantially silicaand iron-free alumina.

The invention will be better understood from thefollowing ydescription taken in conjunction with the accompanying drawings, in which Fig. 1 is a diagrammatic flow sheet of the main features of the method of the invention in which the crude alumina product is puried by the caustic soda digestion treatment, and

Fig. V2 is a similarilow sheet of the method in which the `crude alumina product is puried by the soda-sinter treatment.

The raw or green clay is prepared for calcination in .appropriate steps of sizing `and conditioning. Thus, the clay may be iirst reduced to lumps of about 3A inch in size, ina plurality of stages of size reduction. The sized Amaterial is advantageously conditioned in a pug mill, and then passed through a cutter or shredder. The thus-conditioned clay is dried and calcined in any suitable type of kiln, preferably at a temperature of about 1200 F. with a retention period of about 11/2 hours at that ternperature. 1f the clay is sticky, va certain proportion of dried or calcined clay may be mixed therewith and circulated through the drying and calcining kiln. Dust from the calcining kiln may be collected and returned to the conditioning step following sizing, and a product `cooler may be operatively associated with the kiln. The calcined clay is ground to a nominal particle size of about 65 mesh Tyler Screen Series, advantageously in a 2-stage open circuit ball mill.

The ground calcined clay is leached with hydrochloric acid at a concentration of about 20% HCl. Leaching is advantageously carried out at a temperature of at least 160 F., and preferably as near the boiling temperature vas practical, with mechanical agitation of the slurry for about one hour. Calcination breaks up the aluminum silicate of the clay into alumina and silica, and the alumina is dissolved by the hydrochloric acid in the leaching step. Leaching also dissolves the iron in the clay, so that' leaching produces a solution of aluminum and iron chlorides and a solid residue consisting largely of silica. The solution of chlorides is separated from the solid residue in any suitable type of solids-liquid separator, such as a wet cyclone or lter or a combination thereof, to produce a clarified pregnant solution of aluminum chloride and iron chloride, filter wash liquor and the washed solid residue; the wash liquor being delivered to the HCl absorption tower and the solid residue discharged to waste.A

The pregnant solution of aluminum chloride and iron chloride is evaporated in any suitable manner, as for examplerby vacuum evaporation or the like, to produce a mixed crystal slurry or magma and an aqueous condensate. When, as usual, the crystal slurry contains free Vhydrochloric acid, the aqueous condensate from the evaporator will contain suicient hydrochloric acid to warrant its recovery and return to the process, eg. as lter wash liquor or (if sufficiently concentrated) as yaqueous absorbing mediurn in the HCl absorption tower.

The mixed crystal slurry (usually around 25-30% solids) from the evaporator is treated in a classier (eg. bowl classifier) with the overflow thereof being returned -to the evaporator. The drained crude crystals (usually around 50-60% solids) may advantageously be conditioned in a pug mill preparatory to delivery to the chloride decomposition kiln, as for example in a pug mill type kiln feeder. The kiln is provided with a product cooler, dust collector and means for returning to the kiln dust and (if desired) a certain proportion of the calcined product. In this kiln, the mixed aluminum and iron chloride crystals are dried and calcined. Since, in the flow sheet of Fig. 1, puriiication is to be by the caustic soda digestion treatment, calcnation is carried out at the lower elevated temperature range of 1000 `and 1300" F., and preferably at a temperature of about 1200 F.,-for about 1 hour, in the course of which the chlorides are decomposed (for the most part) into aluminum and iron oxides and hydrogen chloride gas. The calcined product of this heat-treatment constitutes the crude alumina which is to be purified, in this instance, by the caustic soda digestion treatment. The gaseous product of the chloride decomposition kiln is preferably passed through suitable dust recovery equipment; the dust being returned to the kiln and the dust-free gas delivered to the HCl absorption tower. In the absorption tower, hydrogen chloride gas is condensed in an aqueous medium such as an aqueous solution of hydrochloric acid. This may advantageously be done by maintaining the acid absorbing medium near its boiling temperature. By operating the HC1 absorption tower near the boiling'temperature of the acid absorbing'medium, steam will pass through and out of the tower to the stack, while most of the hydrogen chloride will be absorbed and recovered, provided the HCl concentration of the effluent acid does not exceed 20%, which is the concentration at which an aqueous solution of hydrochloric acid is stable, that is the-HC1 concentration of such a solution is unchanged by boiling.

The crude alumina produced in the rst stage is ground, say to nominal mesh, and the ground product is digested in an autoclave, or the like, with caustic soda liquor. Digestion may follow conventional practice iu the art. Thus, the caustic concentration of the digester feed may vary within the range of 100 to 400 g./l. (grams per liter), and usually is within the range of 100 to 200V and preferably around 150 g./l. An excess of caustic soda above the alumina requirement should be used in digestion, such, for example, yas 1.5 to 1.75 Weights of caustic per weight of alumina. The digestion temperature may vary within the range of 275 to 400 F., and the pressure within the range of 50 to 225 pounds per square inch. Iron oxide passes unaected through digestion and is discarded with the insoluble residue. Should there be any silica in the crude alumina, it will form during digestion an insoluble sodium-aluminum-silica complex, which is discarded with the insoluble residue and represents the major alumina and caustic losses, which in the method of the invention are practically negligible because the crude alumina feed lis substantially free of silica.

By suitable successive steps of thickening and filtering the insoluble residue is separated from the pregnant sodium aluminate solution, or green liquor as it is called in the industry. The solution is cooled to atmosphere temperature and pressure, and is then treated with hydrated alumina seed crystals, which upsets the metastable system and causes precipitation of hydrated alumina or aluminum hydroxide. The aluminal content of the green liquor as discharged from digestion may be around of saturation, and when cooled to atmospheric temperature and pressure supersaturation -rnay amount to around 200%. Precipitation is continued until about 50% of the alumina has been -recovered from the green liquor as aluminum hydroxide. The spent mother liquor, together with the required make-up caustic, is heated and returned for digestion of further crude alumina. i

After washing and iiltering, the aluminum hydroxide precipitate is calcined in any suitable type of kiln preferably provided with a cyclone dust collector, and calcine` return means and product cooler, at a temperature within the range of 1700 and 2100 F. and preferably about 1800 F. with a retention period of about one hour at that temperature. The kiln is preferably gas-fired, or may even be oil-iired, rather than coal-iired to avoid ash contamination, The calcine consists of purified alumina,

agen-nea insoluble in hydrochloric acid. Accordingly, in?

practicing the invention, it is desirableto -avoid these unfavorable temperatures, and hencethe clay 'calcining kiln is preferably operated within the temperature rangeof l`l00 and l30`0 F. y The chloride decomposition 'kilnshould in thisf-instance, be operated within the range of 1000 and 1300 F. If the calcining temperatureexceeds about 1300 F., the solubility of alumina in caustic soda is `too low `for economic operation, while ata calciningftempera'ture below :about 1000 F., the Ydecomposition of the chlorides is so imperfect that an objectionable amount --of-chloiide remains in the calcine. This-amounts -to saying that the solubility of alumina in caustic soda decreases as the'ternperature of chloride decomposition is increased, while chloride ldecomposition improves as the calcining teniperature is raised. It is dueto'these considerations that the chloride decomposition kiln is preferably operated at about 1200 F. With a detention period of `about l-hour at that temperature, `at least 99% -of tthe chlorides 'are decomposed with evolution ofthe chlorine -as hydrogen chloride. y

Since the decomposition of the aluminum and iron chlorides can rarely, if ever, be complete at the optimum calcining temperature, the crude alumina will contain Va minor amount of undecomposed chloride, probably `in the -for-m of ferrie chloride. During digestion, `theferric chloride will react with `caustic soda to form insoluble iron hydroxide (Fe(0H)3) and soluble sodium chloride. The iron hydroxide will be discarded -Wi-t-h the insoluble residue, but the sodium chloride will be idissolvedin the sodium aluminate solution, and as spent .green liquor is Thus, clay-caleined-at tem- 'crystallization of spent caustic fs'oda liquor is usedV to cntrol the chloride content "of `fthe electrolytic cell.

The calcined "clay was leached with mechanical agita 'tion for l fhour `a`t -'a temperature `of about 210 F. with aan aqueous-solution of hydrochloric -acid of 20% HC1 repeatedly recycled `sodium chloride will build up inthe caustic soda digestion liquor. ln the eventthat the chlo- --ride decomposition kiln is operated near the .lowertend-'of its aforementioned temperature range, in `order to -i'ncrease the solubility of alumina in caustic soda, lthe accumulation of sodium chloride in the recycled digestion liquor may impair fthe liquor's eectivenes's `and it hence.

will become necessary to remove excess-sodium chloride from the recycling caustic soda liquor.

When the concentration of sodium lchloride in the spent caustic soda liquor exceeds a predetermined maximum, some of the liquor is bled ot and sodium chloride is removed therefrom, and the liquor, vfreed of A:sodium chloride to a desired extent, is returned to the recycling Acaustic: soda liquor. Removal 'of sodium chloride-may 'be by evaporation and crystallization of the spent caustic soda liquor bled off from time to time.

Since make-up hydrochloric acid Vrr'iust be provided in the acid extraction stage, it, would be advantageous in conjunction with the method of lthe invention,"to do 'so by electrolysis of sodium chloride and .reconi-bining the chlorine and hydrogen given olf at the two poles of `the electrolytic cell. Caustic soda would be formed in the cell as a byproduct, 4and would be available -to supply liquor.

y The `crude alumina, ground to nomina-l 1100 mesh, fvv'as digested with caustic soda for aboutfone hour in an fautoclaveffat a :temperature of about 300 F. and pressure 'of :about -1100 pounds per square inch. The concentration Volf the caustic fsoda liquor -fed to the digester kwas about `1-.67 AWeights `of NaOH iper Weight of A1203 rin the crude `alumina fed :to the digester. Digestion, solids- Iliquid separation and cooling of the ipregnant sodium' aluminate Vliquor may, however, be carried out by any -of the presently conventional procedures. Alumina hy- -dr-ate seed -crystals were charged into the cooled and supersaturated pregnant green liquor, and about half of the alumina content of the liquor was precipitated as alumina hydrate (aluminum hydroxide). The ,precipitated aluminahydrate was thickened,was`hed and calcined at"a temperature of 1800" F. 'to produce the final vhigh grade alumina which was substantially free of silica and iron and `admirably adaptedas the feed to electro- 'lytic reduction ffurna'ces.

Referring now Eto the ilowsheet of Fig. `2 in whidh purification of the crude alumina is carried out by the fsoda-sinter t-reatme'nt, the acid '-extraction, -or first stage,

and. preferably Within fthe vrange-of 1700 and 1900 in order to minimize -the amount -of residual chlorine fin :the crude alumina.. -At atcalcining temperatureof `1740!? -F., the crude alumina is substantially freeof chlorine (eg. 0.1% or less), so, for optimum results, the chloride decomposition kiln is operated Within the aforementioned temperature range. In all other respects, the acid extraction treatment is conducted in the manner hereinbefore described in connection with the owsheet of Fig. 1.

The crude alumina, ground to nominal 100 mesh is subjected to a sintering heat treatmentin the presence of sodium carbonate. The sodium carbonate may advantageously be mixed with the crude alumina in the form of a concentrated aqueous solution recovered from the process as hereinafter described. Make-up sodium carbonate may be added dry, to and mixed with the sodasinter charge. The sintering operation'is preferably carried out at a temperature within the range of 1700 and 190()o F. with a retention period of about one hour at thattemperature in any appropriate type of kiln, preferably provided with a pug mill type feeder, cyclone dust collector and return, and product cooler.

In the course of sintering substantially all of th alumina is converted to water-soluble sodium aluminate, while the iron remains in the sinter as insoluble ferrie oxide. The sintered product is leached, preferably with hot wash water from subsequent clarification operations, to dissolve the sodium `aluminate, which is separated from the insoluble sinter residue (largely Fe203) by filtration. If desired, the ltrate may be further clarified in a polishing filter or the like. The clarified sodium aluminate solution is delivered to a carbonation tank for treatment with carbon dioxide gas which may advantageously be obtained from the aluminum hydrate calcining kiln and/ or from 4a natural gas burner or the like.

In the `carbonation tank, sodium aluminate reacts with carbon dioxide to form a precipitate of aluminum hydroxide and water-soluble sodium carbonate. The slurry from the tank is delivered to a hydrate thickener (or other suitable solids-liquid separator) from which the aluminum hydroxide is discharged in the underflow and the sodium carbonate in the overflow. By evaporation the sodium carbonate overow is concentrated to a suitable degree for return to the soda-sinter treatment of crude alumina. The aluminum hydroxide underflow is washed and filtered, and the wash water is utilized in the leaching of soda-sinter product.

The washed aluminum hydroxide is calcined in any suitable ty-pe of kiln, preferably provided with a cyclone dust collector and return means and product cooler, at a temperature within the range of 1700 and 2100 F. and preferably about 1800 F. with a retention period of about one hour at that temperature. The kiln is preferably gas-fired, or may even be oil-lired, rather than coal-fired to avoid ash contamination. The calcine consists of purified alumina, practically silica-free and yanalyzing (dry Weight) 99|-% A1203 and insignificant amounts of Fe203 and NazO. Dust from the gaseous product (kiln gas) of the calcining kiln is recovered in a cyclone and may be returned to the kiln, and the carbon dioxide in the residual gas (after passing the gas through a scrubber tower) is utilized in the aforementioned carbonation tank.

The sodium aluminate produced in the soda-sinter kiln has 4a high degree of Water-solubility. The reaction during leaching ofthe `soda-sinter product is strongly exothermic and leaching is preferably carried voutat a temperature as near the boiling point as practical and at least above 160 F.

The soda-sinter step should be carried out at a temperature of at least 1600"V F. since lower-temperatures do not give good conversion to sodium aluminate. Sintering at a temperature much above 1900 F. gives a more glazed, `less granular product of relatively lower water-solubility. Hence, the preferred temperature of the soda-sinter .step is within the range of 1700 and gas burner.

vcompound is introduced into the circuit as sodium hydroxide, it will be necessary to carbonate it before inclusion in the soda-sinter charge. This may be done by silica-free carbon dioxide gas recovered from the hydrate calcining kiln, from a gas-tired steam boiler stack of the plant, or the like. As hereinbefore mentioned, both sodium hydroxide and hydrochloric acid may be locally made by electrolysis of sodium chloride as conventionallypracticed, and carbon dioxide for carbonating the sodium hydroxide may be` supplied as aforesaid.

, The followingl example illustrates a practice of the invention, in accordance withthe Vllowsheet of Fig.V 2. The acid extraction treatment was carried out as described in the preceding example, except that the chloride decomposition kiln was maintained at a temperature of l700-1800 F. and the detention period of the charge at that temperature was 1 hour. The` calcine (crude alumina) analyzed about 84% A1203, about 16% Fe203,

less than 0.1% residual chlorine, and negligible silica.

The crude alumina was ground to nominal mesh, and mixed with recycled sodium carbonate solution and make-up sodiumcarbonate, in the proportion by weight of about 40% crude alumina, 50% Na2C03 in recycled` solution and 10% make-up Na2C03. This mixture was sintered at a temperature of 1900 F. for 1 hour, in the course of which the alumina was converted to sodium aluminate while iron remained unchanged in the sinter as Fe203. The sinter product analyzed 42.3% A1203, 25.6% Nat-2O, 23.8% Na2CO3 (free) and 8.3% Fe, and Was ground to nominal 100 mesh. The ground sinter was leached with water (filter wash water) at a temperature of about 210 F. in about one-half hours time.

The sodium aluminate filtrate was carbonated by carbon dioxide in the scrubbed gaseous product of a natural The precipitated aluminum hydroxide was thickened, washed and calcined at a temperature of 1800 F. to produce the final high grade alumina, 75% of the alumina content of the original clay being recovered in this product. The thickener overflow and filter wash waters were collected for recovery of sodium carbonate. The sodium carbonate liquors were concentrated by evaporation to an Na2C03 concentration of 212 g./l. and then recycled to the soda-sinter kiln feed.

The final high grade alumina product was substantially free of silica and iron, and admirably adapted as the feed to electrolytic reduction furnaces.

Various types of equipment may be used in carryingV out the individual steps of the method, and apparatus herein specifically described or indicated on the iiowsheets are illustrative and in no manner restrictive. The equipment in the aluminum chloride-ferric chloride and free hydrochloric acid circuits must be resistant Vto corrosive attack, and to this end presently-available. rubber, plastic andceramic types of materials-of-construction are adequate. Aside from pumps, the movements of liquid are relatively slow, and where necessary rubber-lined equipment is readily available therefor. The use of expensive special alloys is a minor item of equipment cost. Drying, calcining and sintering may advantageously be carried out in rotary kilns, although other types of kilns and furnaces may be used. Leaching is carriedout at temperatures that do not require autoclaving and hence open tanks provided with mechanical agitation are entirely satisfactory. Digestion is carried out in autoclaves. Filtration is preferably (but not necessarily) carried out in vacuum type equipment, preceded Where practical by thickening or clarification to reduce as far as possible the areas of filter surface required. Thickening may be carried out in wet cyclones, thickeners, clariiers or the like.

9 Evaporation is preferably carried out in vacuum type evaporators. I claim:

1. The method of producing substantially silicaand iron-free alumina from an iron-containing clay which comprises calcining the clay, dissolving aluminum and iron in the calcined clay by treatment with hydrochloric acid to form aluminum and iron chlorides, converting the aluminum and iron chlorides to aluminum and iron oxides by heat-treatment at a temperature within the range of 1000 and 1300 F., whereby an oxide product which is readily digestible in caustic soda solution is produced, digesting the resulting mixture of aluminum and iron oxides containing a minor amount not exceeding about 1% of residual chlorine in an aqueous caustic soda liquor under pressure and at a hightemperature and thereby producing sodium aluminate with some sodium chloride in solution and an insoluble residue consisting principally of iron oxide, separating said insoluble residue from the nsolution of sodium aluminate and sodium chloride, treating said solution Ato precipitate aluminum hydroxide and thereby producing a spent caustic soda liquor containing some sodium chloride and suitable for digestion of additional aluminum and iron oxides as aforesaid, maintaining the concentration of sodium chloride in said spent caustic soda liquor below a predetermined maximum by removing any excess sodium chloride therefrom, and calcining said aluminum hydroxide precipitate to produce substantially silicaand iron-free alumina.

2. In the method of producing substantially silica-free and iron-free alumina from an iron-containing clay in which an iron-containing crude alumina substantially free of silica is rst produced by calcining the clay, leaching the calcine with hydrochloric acid, separating the siliceous residue from the leach solution, and crystallizing aluminum and iron chlorides from the leach solution, the improvement Which comprises subjecting said chloride crystals to calcination at a temperature within the range of 1000 and 1300 F. and thereby producing an iron-containing crude alumina substantially free of silica which is readily digestible in caustic soda solution, digesting said crude alumina in an aqueous caustic soda liquor under pressure and at high temperature and thereby dissolving the aluminum but not the iron, separating the iron residue from the aluminum-containing liquor, and recovering substantially silica-free and iron-free alumina from said aluminum-containing liquor.

3. The improvement according to claim 2, in which the silica-free and iron-free alumina is recovered from the aluminum-containing liquor by rst precipitating aluminum hydroxide from the liquor and subsequently calcining the aluminum hydroxide at a temperature within the range of 1700 and 2100 F.

4. The improvement according toV claim 3 in which the amount of iron in the clay does not exceed about 6% calculated as Fe203 and the spent caustic soda liquor resulting from the precipitation of aluminum hydroxide contains some sodium chloride Vwhich builds up as the spent liquor is recycled in digesting crude alumina, further characterized in that the concentration of sodium chloride in the recycled spent liquor is maintained below a predetermined maximum by removing excess sodium chloride therefrom.